There's been all sorts of coil guns and rail guns being posted... but here's an interesting alternative. Not really useful, but amusing and effictive as a physics demo. The concept? Conservation of momentum
You'll need some magnets -preferably of the same size, type and somewhat strong (weak magnets do not work as effectivly). You're also going to need some ball bearings that show visible reactions in magnetic fields (aka nickel, coblat and iron/ferrous steel).
You're also going to need a ruler or some sort of straight edge. Movie on next step ;)
Step 1: Setup and Demonstration
I'm using the magnets found inside the magnetix brand of magnetic toy.
Place one magnet on one ball - then place the ball on your straight edge. Attach to the ball (but not to the magnet), one or two additional balls. You can stop here - or repeat this as many times as you want/can.
Take a different ball - and roll it towards the magnet on the first collection of magnet/balls. You should notice that once the ball is inside the magnetic field, it accelerates towards the magnet and makes contact with the magnet/ball cluster. Once contact is made, the last ball(s) separate in the same direction as the original ball.
This phenomina demonstrates the concept of conservation of momentum. If you were to repeat this demonstration without the magnets, you will have the same results with less motion.
The rest of this instructable has more information/examples for those that want to know more.